MC68336/376
QUEUED ANALOG-TO-DIGITAL CONVERTER MODULE
MOTOROLA
USER’S MANUAL
Rev. 15 Oct 2000
8-5
pin is configured as a digital input and the software programmed input/output direction
for that pin is ignored. Refer to D.5.5 information.
8.4.4 Multiplexed Address Output Pins
In non-multiplexed mode, the 16 channel pins are connected to an internal multiplexer
which routes the analog signals into the A/D converter.
In externally multiplexed mode, the QADC allows automatic channel selection through
up to four external 1-of-8 multiplexer chips. The QADC provides a 3-bit multiplexed
address output to the external mux chips to allow selection of one of eight inputs. The
multiplexed address output signals MA[2:0] can be used as multiplex address output
bits or as general-purpose I/O.
MA[2:0] are used as the address inputs for up to four 1-of-8 multiplexer chips (for
example, the MC14051 and the MC74HC4051). Since MA[2:0] are digital outputs in
multiplexed mode, the software programmed input/output direction for these pins in
DDRQA is ignored.
8.4.5 Multiplexed Analog Input Pins
In externally multiplexed mode, four of the port B pins are redefined to each represent
a group of eight input channels. Refer to Table 8-1.
The analog output of each external multiplexer chip is connected to one of the AN[w,
x, y, z] inputs in order to convert a channel selected by the MA[2:0] multiplexed
address outputs.
8.4.6 Voltage Reference Pins
VRH and VRL are the dedicated input pins for the high and low reference voltages. Sep-
arating the reference inputs from the power supply pins allows for additional external
filtering, which increases reference voltage precision and stability, and subsequently
contributes to a higher degree of conversion accuracy. Refer to Table A-11 and Table A-12 for more information.
8.4.7 Dedicated Analog Supply Pins
VDDA and VSSA pins supply power to the analog subsystems of the QADC module.
Dedicated power is required to isolate the sensitive analog circuitry from the normal
Table 8-1 Multiplexed Analog Input Channels
Multiplexed Analog Input
Channels
ANw
Even numbered channels from 0 to 14
ANx
Odd numbered channels from 1 to 15
ANy
Even channels from 16 to 30
ANz
Odd channels from 17 to 31